Archive for October, 2010

In a departure from the traditional semi-rigid rotor system design, Frank Robinson used a coning hinge on each blade when designing the R22 in the 1970s. When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). Although some mistakenly refer to this as a flapping hinge, it is used for blade coning. Previous rotor systems used a fixed coning angle built into each blade. Robinson’s design allows the coning angle to vary according to different conditions such as rotor speed, acceleration, and weight.

Coning via blade bending places a high stress load at the blade’s root. The coning hinge relieves this stress, reducing the amount of reinforcing required and making for a lighter, easier to manufacture rotor blade. Additionally, the reduced bending of the rotor blade at its root allows the pitch-change axis to be better aligned with the blade’s centerline. This reduces the forces across the hub, pitch change bearings, and the rotor blades and, as such, decreases shake and feedback in the cyclic control.

Known as a tri-hinge rotor hub (the third hinge is called a teetering hinge and is typically the only hinge in a semi-rigid rotor system) Robinson was granted a patent for it in 1978. The same hub design (although larger in size) is used on the R44 and the new turbine-powered R66.

The quest for a device that a person could strap on his back, lift off from his backyard, and fly around started in the 1940s. One of the more popular ideas came from Seattle, Washington, inventor Horace Pentecost. He actually designed and built a small backpack with a coaxial rotor system powered by a 20-hp two-cylinder air-cooled engine. The assembly weighed about 90 lbs and had a 12-foot rotor diameter. An overhead cyclic control also controlled yaw with a twist grip and moved vertically to control collective pitch.

Dubbed the “Hoppi-Copter” it was plagued with several issues like getting away from the fuselage quickly in an emergency and using the pilot’s legs as landing gear (too unstable). To solve some of these problems Pentecost built a second model with a 40-hp engine and simple tripod assembly for landing gear. Even so, the tiny aircraft proved unstable and too difficult to fly. Thinking the machine also had a military appeal, he continued working on improving the design.

However, shortly into the test program a major problem arose. Early on when he had formed his company he had given 10 percent of it to his lawyer in exchange for setting up the corporation. Pentecost and his wife divorced giving her half of his 90-percent ownership. His ex-wife and lawyer soon got together and Pentecost was pushed out of the company he started. In 1956 the company was sold to a group of investors from Washington, D.C. Their idea was to raise enough capital to certify the tiny helicopter by selling 300,000 shares of the company at $1 each. Ultimately, they were unsuccessful and the original Hoppi-copter ended up at the Smithsonian National Air and Space Museum where it remains in storage or out on loan.

A maximum performance takeoff is used to climb at a steep angle to clear barriers in the departure flight path. To perform this maneuver successfully a pilot must consider the wind velocity, temperature, altitude, gross weight, center-of-gravity location, and other factors affecting performance of the helicopter.

The textbook procedure is this: After performing a hover power check to determine if there is sufficient power available, position the helicopter into the wind and begin by getting the helicopter light on the skids. Pause to neutralize all aircraft movement. Slowly increase the collective and position the cyclic to break ground in a 40-knot attitude. This is normally about the same attitude as when the helicopter is light on the skids. Continue to slowly increase the collective until the maximum power available is reached. Keep in mind this large collective movement requires a substantial increase in pedal movement to maintain heading. Use the cyclic, as necessary, to control movement toward the desired flight path and, therefore, climb angle during the maneuver. Maintain rotor rpm at its maximum, and do not allow it to decrease since you would probably have to lower the collective to regain it. Maintain these inputs until the helicopter clears the obstacle then establish a normal climb attitude and reduce power. As in any maximum performance maneuver, the techniques you use affect the actual results. Smooth, coordinated inputs coupled with precise control allow the helicopter to attain its maximum performance. Also, the helicopter will most likely be inside the shaded area of the height-velocity diagram where there is very little energy available to perform an autorotation in the event of an engine failure. Using maximum power would decrease the time the helicopter is exposed to a low energy situation.

However, other pilots I have talked to prefer a slightly different technique. Instead of using maximum power available, use the minimum power necessary to safely climb and clear the obstacle. The theory being that in the event of an engine failure the less power the pilot is using the more rotor rpm will be recoverable to help in the autorotation. Also, demanding high power might increase the probability of a part failure at a critical time. I would be interested in hearing comments on the two theories.